It has been the practice in running strings into a well through a ram type blowout preventer connected at the wellhead to support the string by closing the blowout preventer rams on the string immediately below a collar or an upset shoulder in the string so that the rams support the string and the string can be released from the travelling block so that other operations, such as adding a section to the string, may be performed.
Efforts have been made to incorporate gripping elements into the face of the face of the ram. An example of this is shown in the prior application Ser. No. 06/853,601, filed Apr. 18, 1986. This application discloses a ram type blowout preventer which is intended to close on two strings at the same time, sealing against both strings and bringing gripping elements into engagement with the strings so that the strings are supported on the rams.
Another prior structure is disclosed in the U.S. Pat. No. 4,043,389. This structure is a ram type blowout preventer having an upper set of rams to seal against the string extending through the preventer and a lower unit which includes a slip bowl secured to each ram arm which extends into the guideways from the pressure responsive means and a plurality of slips positioned on the tapered front face of each bowl, with the two outer slips being secured by resilient straps to the bowl and the other slips being interengaged with each adjacent slip by a connecting member. In this structure, the bowls function as the rams of a normal blowout preventer and when moved inward into the central bore function to bring the slips into engagement with the string. As the slips engage the string and assume the load they are forced downward and the taper on the face of the bowl causes them to be wedged tighter against the string.
Two other U.S. Pat. Nos. 2,542,302 and 3,017,931 disclose rams having slip assemblies mounted thereon and means for moving the rams into the bore of the wellhead to bring the slip assemblies into supporting engagement with a string extending therethrough.